French police say the Iraqi-born British engineer Saad al-Hilli, who was gunned down with three other people in an Alpine beauty spot a year ago, had an unusual amount of information related to his work with a satellite company.

They are trawling through hours of recordings between the father-of-two and relatives and friends, which they say sheds light on the "violent" dispute between Hilli and his older brother Zaid over their father's will.

Officers outlined the two main lines of inquiry – industrial espionage and a family row that ended in violence – at a joint press conference between British and French police at Annecy, a year and a day after Saad, his wife, Iqbal, and her mother, Suhaila, were shot dead near the village of Chevaline.

However, police said they still had no idea who the killer might be except that he was an "experienced gunman".

"We don't know who they are except that they could hit a moving target and change cartridges. It is clear this was a hardened gunman."

The investigation into the shootings has led French detectives, working with their British counterparts, to make inquiries worldwide, including Iraq, the former Yugoslavia and Switzerland.

Apart from Hilli's wife, Iqbal, 47, and her mother, Suhaila al-Allaf, 74, a passing cyclist, Frenchman Sylvain Mollier, 45, a father-of-three who appeared to have stumbled across the scene, was also murdered.

The Hillis' two daughters were the only survivors of the shooting. The older girl, Zainab, aged seven, was shot in the shoulder and beaten around the head, leaving her with a fractured skull. Her sister, Zeena, four, escaped uninjured after hiding for eight hours in the rear footwell of the family's BMW 5-series estate under the skirt of her dead mother.